1. Field of the Invention
The present invention relates to a planar optical circuit that can be applied to a wavelength division multiplexing transmission system.
2. Description of the Background Art
An optical fiber amplifier amplifies signal light in an optical fiber. The optical fiber amplifier installed on an optical transmission line is equipped with an optical fiber for amplification, which also functions as the optical transmission line, and a pump light supplying means which supplies the optical fiber for amplification with the pump light. When signal light is input into the optical fiber for amplification to which pump light is supplied, the signal light is amplified and output. As for the optical fiber amplifier generally used heretofore, there are a rare-earth element doped fiber amplifier which uses stimulated emission phenomenon of a rare earth element, e.g. erbium, and a Raman amplifier which uses Raman amplification phenomenon by the stimulated Raman scattering.
In a Raman amplifier, an amplification wavelength band can be designed at any wavelength range by selecting a suitable wavelength for pumping light. Also, if pump light having a plurality of different wavelengths is supplied to the optical fiber for amplification, signal light can be amplified in the respective wavelength band adjusted to the amplification wavelength band determined by each pump light wavelength.
In recent years, the development and use are advanced with respect to the Wavelength Division Multiplexing (WDM) transmission system that can transmit signal light consisting of a plurality of signals having different wavelengths in an optical transmission line. When an optical amplifier is applied to such WDM transmission system, it is important to amplify the respective signals simultaneously with an equal gain to output them with constant power within a certain range. Therefore, in the WDM transmission system using an optical amplifier, an optical filter for canceling the wavelength dependence of intrinsic gain spectrum is used as a gain equalizer.
For example, techniques (gain equalization technology) for decreasing wavelength dependence in the gain of an optical amplifier by an optical filter using Mach-Zehnder interferometer are described in Literature 1 xe2x80x9cK. Inoue, et al., xe2x80x9cTunable Gain Equalization Using a Mach-Zehnder Optical Filter in Multistage Fiber Amplifiersxe2x80x9d, IEEE Photonics Technology Letters, Vol. 3, No. 8, pp. 718-720 (1991)xe2x80x9d and in Literature 2 xe2x80x9cG. H. B. Thompson, et al., xe2x80x9cPlanar Waveguide Filters for Dynamic Equalization of EDFA Spectraxe2x80x9d, ECOC""99, pp 1-320-1-321(1999).xe2x80x9d
These technologies are such that the wavelength dependence of loss in an optical filter is adjusted by adjusting the temperature of each arm of the Mach-Zehnder interferometer according to input signal power and thereby the wavelength dependence of intrinsic gain spectrum is canceled. Such a gain equalization technology is important for increasing transmission capacity of the WDM transmission system. As shown in FIG. 1 of Literature 1, an optical filter is placed outside the amps 1, 2, and 3 and hence pump light does not pass through the optical filter. Therefore, loss at a pump light wavelength is not considered in the optical filter.
An object of the present invention is to provide a planar optical circuit which can equally adjust the power of each signal of signal light that is wavelength-multiplexed and which can allow light of a second wavelength band different from the signal light wavelength band to pass at low loss, and to provide an optical transmission system using the planar optical circuit.
The planar optical circuit according to the invention has a substrate and an optical waveguide path formed on the substrate. This optical circuit comprises a signal light input end, signal light output end, and a filter disposed between the signal light input end and the signal light output end. The filter causes variably controllable wavelength dependent loss to signal light and is constituted such that light in a second wavelength band, which is different from a signal light wavelength band, is output at the other end with substantially constant loss.
This filter may be constituted by connecting in series a wavelength selecting circuit for defining a wavelength range in which wavelength dependent loss appears and an attenuation slope control circuit for defining the inclination of wavelength dependent loss relative to a wavelength.
In such a case, the wavelength selecting circuit may have a main waveguide path for connecting the signal light input end and signal light output end and a secondary waveguide path which is optically coupled with main waveguide path through at least two optical couplers and which constitutes a Mach-Zehnder interferometer in combination with the main waveguide path and the optical couplers. Also, the attenuation slope control circuit may have a main waveguide path, which connects the signal light input end and signal light output end, a secondary waveguide path which is optically coupled with the main waveguide path through at least two optical couplers and which constitutes a Mach-Zehnder interferometer in combination with the main waveguide path and the optical couplers, and a means for adjusting the temperature of the main waveguide path or secondary waveguide path. The filter may be composed of a plurality of wavelength selecting circuits and a plurality of attenuation slope control circuits, which are connected in series at a plurality of steps.
The signal light wavelength band may be designed to include a plurality of different wavelengths, and the second wavelength band may be arranged such that signal light in a signal light wavelength band can be Raman-amplified.
In addition, herein provided is an optical transmission system that is equipped with an optical transmission line for transmitting signal light and a planar optical circuit according to the present invention for causing wavelength dependent loss to signal light. The planar optical circuit is installed at a given position on the optical transmission line.
The present invention is further explained below by referring to the accompanying drawings. The drawings are provided solely for the purpose of illustration and are not intended to limit the scope of the invention.